Wind
Amazing that people do not know you most of the time lose with wind as Nick points out.
Again the mathematician in me can explain you the second order term in the equations etc etc,
t1 = dist / (V + w) (go with wind in the tail)
t2 = dist / (V - w) (come back with wind on the nose)
average time (after some arithmetic)
(t1+t2)/2 = dist * V / (V.V - w.w) is always greater than dist / V
or let me just refer to the correction made by Veeany to 407 post.
So I have a general tendency to hope for litlle wind, certainly if I have to plan for a round trip.
d3